Transmission or exchange of data, i.e., information is a frequent requirement in many areas. For example, it may be desirable to collect measured values relating to physical variables from a variety of measuring points, which are distributed in space. In addition to the possibility of connecting the individual sensors at the measuring points by means of cables to a central data collecting device, radio-based approaches have become increasingly important, mainly for reasons of cost and flexibility. Some types of such devices are referred to as sensor networks, among other things. A sensor network is a system of spatially distributed sensor nodes, which interact independently with one another and also with the existing infrastructure by radio—depending on the application. This serves to acquire, process, forward and supply information and/or data from the physical world. Sensor networks may differ in the type of networking, the topology and the direction of data flow, for example.
FIG. 1 shows as an example a schematic topological view of a wireless sensor network 1. The sensor network 1 comprises sensor nodes 4, anchor nodes and/or router nodes 5 and, in the example shown here, a network transfer, i.e., gateway node 6. The sensor network 1 is connected to a main network, i.e., a backbone network 2 via the gateway node 6. An analysis system may also be connected as a back-end system 3 to the backbone network 2.
In general, the topology of a sensor network cannot be determined in advance and/or may change during operation. One possibility of responding to the variability of the topology of the sensor network consists of leaving the organization of the communication, i.e., the network topology to the nodes forming the network. The communication, i.e., the network topology is then organized automatically by the network. Especially in the case of radio-based sensor networks but also with other transmission technologies that are used, it may happen that a transmitter does not have enough range to be able to reliably send the data to be transmitted to a receiver, which is at a distance spatially from the transmitter. In this case, it may be provided that, with many network topologies, the data packets are forwarded to the receiver via intermediate nodes (multi-hop communication). A receiver therefore need not be in direct (radio) range of the original transmitter.
In many applications, the sensor nodes are usually battery operated, so wireless communication is a special challenge. Depending on the application, the requirements with regard to the allowed latency, the data rate that may be used, the number of nodes and the desired topology may be elucidated and weighed against the available energy.
There are the following main reasons for electric power consumption in communication:                “Idle listening” (ready-to-receive status): a node has activated the receiver and is listening for nonexistent messages that might be sent. This is usually one of the main sources or causes of electric power consumption.        “Overhearing”: a node receives messages not intended for it.        “Collisions”: two packets sent simultaneously are destructively superimposed at the receiver and may be repeated.        “Control overhead” (expenditure for control and/or coordination): the protocol header information involves additional time for its transmission and thus consumes energy.        
Because of the limited energy, especially in battery-operated devices, some protocols for multi-hop communication in wireless sensor networks attempt to coordinate their transmission and receiving processes to thereby achieve the shortest possible activity cycles.
A number of protocols have introduced a timing coordination through a central instance (master) in the system. Examples of this include IEEE 802.15.4 in the so-called beacon mode or the s-net protocol of the Fraunhofer Institute for Integrated Circuits, which is also described in the European Patent EP 1 815 650 B1.
FIG. 2 shows schematically a network structure of the sensor network 1 and is cited to illustrate an example of tree-type propagation of a synchronization signal and/or a coordination signal in the sensor network 1. The sensor network 1 is structured according to a self-organizing tree topology. As mentioned above, a timing synchronization may be used to enable an energy-saving communication. For the timing synchronization, the synchronization nodes will periodically transmit beacon signals. The nodes are organized independently along a tree structure. Except for the master node 7, each node has a parent node and may either have no child node, one child node or multiple child nodes. Dynamic adjustments in network structure may occur in particular when there are changes in the spatial position and/or the transmission conditions between two or more nodes.
The main node, i.e., master node 7 belongs to a layer group 0 and periodically transmits a beacon signal. For example, the master node 7 may be defined as the master node on the basis of a configuration by the user or the network administrator of the sensor network 1. Another possibility is for the master node 7 to be the first node activated within the sensor network 1, for example, and thus transmitted first as a beacon signal. In the example illustrated in FIG. 2, there are six additional nodes within the reception range of the master node 7, namely the end node and/or sensor nodes 4 with the node identification numbers 1, 18 and 20 indicated within the circles as well as the anchor nodes 5 with the node identification numbers 2, 4 and 5. The six above-mentioned nodes with the node identification numbers 1, 2, 4, 5, 18 and 20 belong to a layer group 1 because they are capable of receiving the beacon signal directly from the master node 7. The beacon signal of the master node 7 contains at least one item of information that was sent from a node of the layer group 0. The anchor nodes 5 with the node identification numbers 2, 4 and 5 then in turn emit beacon signals but they are typically offset in time in relation to the beacon signal of the master node 7. The beacon signal emitted by the anchor node having the node identification number 2 is received by the anchor node having the node identification numbers 3 and 6, for example, which are thus assigned to the layer group 2. The beacon signal of the anchor node having the node identification number 4 is received by two other anchor nodes having the node identification numbers 8 and 12; the data signal emitted by the anchor node having the node identification number 5 is received by the end nodes having the node identification numbers 13 and 14. As a representative example, the anchor node having the node identification number 7 will now be considered. The anchor node having the node identification number 7 is within the range of the anchor node having the node identification number 3 and belongs to the layer group 3. Since another external end node having the node identification number 17 is also within the reception area of a node having the node identification number 7, the node no. 17 is assigned to the layer group 4 and the node no. 7 is a parent node for the node no. 17.
FIG. 2 thus shows that the master node 7 (node no. 0) transmits its information and/or synchronization signals as nodes of the layer 0. These signals are received by the node of the layer 1, which in turn send their information and/or synchronization signals as nodes to the layer 1. These signals are received by nodes of the layer 2, etc. A frame structure having time slots for the (periodic) send/receive activities of the individual nodes is often defined for sending the information and/or synchronization signals. If two nodes whose signals can both be received by a third node are using the time slot for sending, a collision may occur at the third node, so that the third node cannot reliably receive either of the two signals that are sent.
It would be desirable if the effects of such a situation were less serious for the third node, which does not have an opportunity to participate in the communication with the sensor network 1 because it does not have an opportunity for synchronization with the remaining network. Alternatively or additionally, it would also be desirable for the situation described above to be detectable and correctable, if possible.